Variable displacement refrigerant compressors have been employed in engine driven automotive air conditioning systems in order to reduce engine load variations associated with compressor cycling. In the system manufactured by the Harrison Radiator Division of General Motors Corporation, for example, the compressor displacement is controlled by regulating the compressor crankcase pressure To this end, a pneumatic control valve integral to the compressor variably connects the compressor crankcase to the inlet (suction) and outlet (discharge) chambers of the compressor. In an electronic version of the control, the control valve is mechanized with a solenoid valve positioned to achieve the ratiometric control. The valve may be linearly positioned by controlling the solenoid current, or pulse-width-modulated at a variable duty cycle to alternately connect the crankcase to the inlet and outlet chambers.
As with fixed displacement compressors, internal lubrication is provided by a small amount of oil suspended in the refrigerant The amount of refrigerant in the system, referred to herein as the refrigerant charge, therefore determines the degree of compressor lubrication as well as the cooling performance of the system. If a significant portion of the refrigerant escapes, compressor lubrication may be insufficient and continued operation under such conditions may severely damage the compressor.
Various arrangements have been proposed for detecting the refrigerant charge in an air conditioning system and for taking the appropriate protective action when a low charge condition occurs. One such system for a fixed displacement compressor is disclosed in the Burnett U.S. Pat. No. 4,463,576, et al. issued Aug. 7, 1984, and assigned to the assignee of the present invention. In that system, the compressor is cycled on and off as a function of the refrigerant vapor pressure, and a low charge condition is indicated when a specified number of successive short duration on-periods occur. This method is effective in the protection of cycled fixed displacement compressors, but is not applicable to variable displacement compressors since variable displacement compressors are not cycled on and off in normal operation. Various refrigerant level measuring devices have also been proposed.